An eccentric oscillating type gear transmission is known. An eccentric oscillating type gear transmission can obtain a larger reduction ratio than a typical gear transmission provided with a plurality of spur gear trains. In this type of gear transmission, an external gear rotates eccentrically relative to an internal gear while meshing with the internal gear. Eccentric oscillating type gear transmissions include a type in which the external gear rotates eccentrically, and a type in which the internal gear rotates eccentrically. Below, to simplify the description, the type in which the external gear rotates eccentrically will be described. In this type of gear transmission, a crankshaft is supported on a member called a carrier. The external gear is engaged with the crankshaft. The crankshaft has an eccentric body for eccentrically rotating the external gear. The eccentric body fits with the external gear. The external gear can be said to be supported on the carrier via the crankshaft.
The internal gear is formed on the inner side of a case. When rotational torque of a motor is applied to the crankshaft, the external gear rotates eccentrically relative to the internal gear. The number of teeth of the external gear differs from the number of teeth of the internal gear. Consequently, when the external gear rotates eccentrically while meshing with the internal gear, the external gear rotates relative to the internal gear in accordance with the difference in number of teeth of the external gear and the internal gear. Since the carrier is supporting the external gear, the carrier also rotates relative to the internal gear. Consequently, the carrier rotates relative to the case. In situations where the case of the gear transmission is fixed to a base member, the carrier corresponds to an output member of the gear transmission. Since the reduction ratio in this situation is determined by the “difference in number of teeth/(number of teeth of internal gear+1)”, a large reduction ratio can be obtained. Moreover, in the type of gear transmission in which the internal gear rotates eccentrically, the external gear corresponds to the output member of the gear transmission. Further, in the present specification, a large denominator of the reduction ratio (a small absolute value of the reduction ratio) is expressed as the “reduction ratio being large”.
When two eccentric oscillating type gear transmissions are connected in series, a larger reduction ratio can be obtained. Examples of this type of gear transmission are disclosed in Japanese Patent Application Publication No. 2006-317009 and Japanese Patent Application Publication No. H1-242850. In the description below, Japanese Patent Application Publication No. 2006-317009 is called Patent Document 1, and Japanese Patent Application Publication No. H1-242850 is called Patent Document 2. The gear transmissions of Patent Document 1 and Patent Document 2 comprise two eccentric oscillating type gear transmissions (reduction units). An output member (carrier) of a first reduction unit of both the gear transmissions is connected with an input member (crankshaft) of a second reduction unit.